1. Field of the Invention
The present invention relates to a defrosting system for a heat exchanger which is utilized in an air conditioner and so on, in particular to an improved control in defrosting operation for the heat exchanger.
2. Discussion of Background
In general, the heat exchanger is utilized in an air conditioner, a refrigerator, a refrigerated show case and so on.
The application of the heat exchanger will be described in reference to a conventional defrosting system in the air conditioner which is disclosed in Japanese Unexamined Pat. Publication No. 61530/1981.
FIG. 5 is a block diagram showing the conventional defrosting system. Reference numeral 1 designates an outdoor heat exchanger. Reference numeral 2 designates a temperature detector which is arranged in close proximity to the outdoor heat exchanger 1. Reference numeral 3 designates a defrosting start/end signal generating device. Reference numeral 4 designates a timer for controlling a prohibitive time against defrosting operation, which is connected to the defrosting start/end signal generating device. Reference numeral 5 designates a timer for counting the time required to complete the defrosting operation. The timer 5 is connected to the generating device 3 and the timer 4, and counts the time for which the generating device 3 is outputting a defrosting operation signal.
In operation, when the air conditioner carries out heating operation, the outdoor heat exchanger 1 functions as an evaporator. The formation of frost on the heat exchanger causes the evaporation temperature to drop. The evaporation temperature is detected by the temperature detector 2. When the detected temperature drops to a predetermined temperature or below (i.e. a great amount of frost is formed on the heat exchanger, reducing its efficiency), a temperature signal is input from the temperature detectror 2 to the defrosting start/end signal generating device 3. The timer 5 for counting the time required to complete the defrosting operation determines the time for which the restarted heating operation must be continued after the completion of the previous defrosting operation, and counts the time for which the heating operation is being carried out. When the counted time reaches the determined time, the timer 5 outputs a time up signal to the defrosting start/end signal generating device 3. The generating device 3 outputs a defrosting start signal when it has received the temperature signal and the time up signal.
The timer 4 is used to set the next prohibitive time against the defrosting operation depending on the time required to complete the last defrosting operation, which is counted by the timer 5. The timer 4 sets the next prohibitive time in such manner that when the time required to complete the last defrosting operation is short, the next prohibitive time is lengthened (because the formation of frost can be considered to be small), and when the time required to complete the last defrosting operation is long, the next prohibitive time is shortened (because much frost is likely to be formed on the heat exchanger). A defrosting end signal is output by the defrosting start/end signal generating device 3 when during the defrosting operation, the temperature detector 3 detects a temperature having a predetermined value or above (i.e. it detects a temperature not lower than the predetermined value that can not obtained when the frost remains).
Such structure of the conventional defrosting system creats problems wherein the defrosting system can be significantly affected by weather conditions to be prevented from carrying out its proper defrosting performance and the outdoor heat exchanger 1 can not restore the ability as the evaporator. Specifically, when humidity around the outdoor heat exchanger 1 becomes high during the heating operation wherein a longer prohibitive time is set, the amount of frost which has been formed on the heat exchanger until lapse of the prohibitive time can be greater than expected, thereby requiring to greatly lengthen the next defrosting time. At the worst, the defrosting operation fails to have fully defrosted the heat exchanger, and the frost which has not been eliminated can remains as ice.
It is an object of the present invention to eliminate the problems of the conventional defrosting system and to provide a defrosting system for a heat exchanger capable of keeping stable defrosting performance even if weather conditions such as humidity change, and of maintaining high efficiency of the heat exchanger.
The foregoing and the other objects of the present invention have been attained by providing a defrosting system for a heat exchanger comprising a temperature detector adapted to be arranged in close proximity to a heat exchanger; a temperature memory for storing temperature data detected by the temperature detector; a first timer for counting a defrosting prohibitive time; a second timer for counting the time required to complete the defrosting operation; a switching means for switching the flowing direction of a refrigerant to the heat exchanger; and a central processing unit for controlling the temperature memory, the first and second timers, and the switching means, and further carrying out arithmetic manipulations; wherein the central processing unit sets the next prohibitive time depending on the time counted by the second timer; and when a second temperature which is detected by the temperature detector after normal operation requested by a user has restarted and after a predetermined minimum defrosting prohibitive time has passed, drops by a predetermined difference in temperature from a first temperature which is detected by the temperature detector after the normal operation has restarted and after a predetermined time has passed, and when the second temperature is a predetermined temperature or below; the central processing unit carries out the defrosting operation even if the prohibitive time has not passed.
In accordance with the present invention, the next prohibitive time is set to be modified in accordance with the defrosting time. In addition, when the evaporation temperature of the heat exchanger drops by a predetermined value from the maximum value after normal operation (heating or cooling operation) starts, the defrosting operation is initiated. As a result, when the amount of frost on the heat exchanger is small and the defrosting operation ends in a short time, the interval between the last defrosting operation and the next one can be prolonged, allowing the heat exchanger to maintain effective capability. In addition, when the amount of frost on the heat exchanger has suddenly increased due to the change of air conditions and so on during a long porhibitive time against the defrosting operation, the defrosting operation is carried out even if the prohibitive time has not passed yet, preventing the capability of the heat exchanger from lowering and the frost from remaining after the defrosting operation. In this way, the present invention offers advantage of providing the defrosting system having high reliability and good efficiency.